[MINOR] lb_map: reorder code in order to ease integration of new hash functions

[MINOR] lb_map: reorder code in order to ease integration of new hash functions

We need to remove hash map accesses out of backend.c if we want to
later support new hash methods. This patch separates the hash computation
method from the server lookup. It leaves the lookup function to lb_map.c
and calls it with the result of the hash.